Use /etc/DISTR-release with DISTR= lsb (Ubuntu), redhat, gentoo, mandrake, sun (Solaris), and so on. See also /etc/issue.

# uptime # Show how long the system has been running + load
# hostname # system's host name
# hostname -i # Display the IP address of the host. (Linux only)
# man hier # Description of the file system hierarchy
# last reboot # Show system reboot history

Encrypted passwords are stored in /etc/shadow for Linux and Solaris and /etc/master.passwd on FreeBSD. If the master.passwd is modified manually (say to delete a password), run # pwd_mkdb -p master.passwd to rebuild the database.

To temporarily prevent logins system wide (for all users but root) use nologin. The message in nologin will be displayed (might not work with ssh pre-shared keys).

Solaris

The following values in /etc/system will increase the maximum file descriptors per proc:

set rlim_fd_max = 4096 # Hard limit on file descriptors for a single proc
set rlim_fd_cur = 1024 # Soft limit on file descriptors for a single proc

Runlevels

Linux

Once booted, the kernel starts init which then starts rc which starts all scripts belonging to a runlevel. The scripts are stored in /etc/init.d and are linked into /etc/rc.d/rcN.d with N the runlevel number.The default runlevel is configured in /etc/inittab. It is usually 3 or 5:

# grep default: /etc/inittab
id:3:initdefault:

The actual runlevel can be changed with init. For example to go from 3 to 5:

# init 5 # Enters runlevel 5

0 Shutdown and halt

1 Single-User mode (also S)

2 Multi-user without network

3 Multi-user with network

5 Multi-user with X

6 Reboot

Use chkconfig to configure the programs that will be started at boot in a runlevel.

FreeBSD

The BSD boot approach is different from the SysV, there are no runlevels. The final boot state (single user, with or without X) is configured in /etc/ttys. All OS scripts are located in /etc/rc.d/ and in /usr/local/etc/rc.d/ for third-party applications. The activation of the service is configured in /etc/rc.conf and /etc/rc.conf.local. The default behavior is configured in /etc/defaults/rc.conf. The scripts responds at least to start|stop|status.

Reset root password

Linux method 1

At the boot loader (lilo or grub), enter the following boot option:

init=/bin/sh

The kernel will mount the root partition and init will start the bourne shellinstead of rc and then a runlevel. Use the command passwd at the prompt to change the password and then reboot. Forget the single user mode as you need the password for that.If, after booting, the root partition is mounted read only, remount it rw:

FreeBSD method 1

On FreeBSD, boot in single user mode, remount / rw and use passwd. You can select the single user mode on the boot menu (option 4) which is displayed for 10 seconds at startup. The single user mode will give you a root shell on the / partition.

# mount -u /; mount -a # will mount / rw
# passwd
# reboot

Unixes and FreeBSD and Linux method 2

Other Unixes might not let you go away with the simple init trick. The solution is to mount the root partition from an other OS (like a rescue CD) and change the password on the disk.

Boot a live CD or installation CD into a rescue mode which will give you a shell.

To modify and rebuild the kernel, copy the generic configuration file to a new name and edit it as needed (you can also edit the file GENERIC directly). To restart the build after an interruption, add the option NO_CLEAN=YES to the make command to avoid cleaning the objects already build.

# make buildworld # Build the full OS but not the kernel
# make buildkernel # Use KERNCONF as above if appropriate
# make installkernel
# reboot
# mergemaster -p # Compares only files known to be essential
# make installworld
# mergemaster -i -U # Update all configurations and other files
# reboot

For small changes in the source you can use NO_CLEAN=yes to avoid rebuilding the whole tree.

Repair grub

So you broke grub? Boot from a live cd, [find your linux partition under /dev and use fdisk to find the linux partion] mount the linux partition, add /proc and /dev and use grub-install /dev/xyz. Suppose linux lies on /dev/sda6:

Start the process with a defined priority with nice. Positive is “nice” or weak, negative is strong scheduling priority. Make sure you know if /usr/bin/nice or the shell built-in is used (check with # which nice).

While nice changes the CPU scheduler, an other useful command ionice will schedule the disk IO. This is very useful for intensive IO application (e.g. compiling). You can select a class (idle – best effort – real time), the man page is short and well explained.

The last command is very useful to compile (or debug) a large project. Every command launched from this shell will have a lover priority. $$ is your shell pid (try echo $$).FreeBSD uses idprio/rtprio (0 = max priority, 31 = most idle):

Background/Foreground

When started from a shell, processes can be brought in the background and back to the foreground with [Ctrl]-[Z] (^Z), bg and fg. List the processes with jobs. When needed detach from the terminal with disown.

No straight forward way to re-attach the process to a new terminal, try reptyr (Linux).Use nohup to start a process which has to keep running when the shell is closed (immune to hangups).

# nohup ping -i 60 > ping.log &

Top

The program top displays running information of processes. See also the program htop from htop.sourceforge.net (a more powerful version of top) which runs on Linux and FreeBSD (ports/sysutils/htop/). While top is running press the key h for a help overview. Useful keys are:

u [user name] To display only the processes belonging to the user. Use + or blank to see all users

Permissions

Change permission and ownership with chmod and chown. The default umask can be changed for all users in /etc/profile for Linux or /etc/login.conf for FreeBSD. The default umask is usually 022. The umask is subtracted from 777, thus umask 022 results in a permission 0f 755.

Mount a FreeBSD partition with Linux

Find the partition number containing with fdisk, this is usually the root partition, but it could be an other BSD slice too. If the FreeBSD has many slices, they are the one not listed in the fdisk table, but visible in /dev/sda* or /dev/hda*.

Mount an SMB share

Suppose we want to access the SMB share myshare on the computer smbserver, the address as typed on a Windows PC is \\smbserver\myshare\. We mount on /mnt/smbshare. Warning> cifs wants an IP or DNS name, not a Windows name.

Solaris and FreeBSD

Create and burn an ISO image

This will copy the cd or DVD sector for sector. Without conv=notrunc, the image will be smaller if there is less content on the cd. See below and the dd examples.

# dd if=/dev/hdc of=/tmp/mycd.iso bs=2048 conv=notrunc

Use mkisofs to create a CD/DVD image from files in a directory. To overcome the file names restrictions: -r enables the Rock Ridge extensions common to UNIX systems, -J enables Joliet extensions used by Microsoft systems. -L allows ISO9660 filenames to begin with a period.

Linux

Also use cdrecord with Linux as described above. Additionally it is possible to use the native ATAPI interface which is found with:

# cdrecord dev=ATAPI -scanbus

And burn the CD/DVD as above.

dvd+rw-tools

The dvd+rw-tools package (FreeBSD: ports/sysutils/dvd+rw-tools) can do it all and includes growisofs to burn CDs or DVDs. The examples refer to the dvd device as /dev/dvd which could be a symlink to /dev/scd0 (typical scsi on Linux) or /dev/cd0 (typical FreeBSD) or /dev/rcd0c (typical NetBSD/OpenBSD character SCSI) or /dev/rdsk/c0t1d0s2 (Solaris example of a character SCSI/ATAPI CD-ROM device). There is a nice documentation with examples on the FreeBSD handbook chapter 18.7http://www.freebsd.org/handbook/creating-dvds.html.

The file based image can be automatically mounted during boot with an entry in /etc/rc.conf and /etc/fstab. Test your setup with # /etc/rc.d/mdconfig start (first delete the md0 device with # mdconfig -d -u 0).Note however that this automatic setup will only work if the file image is NOT on the root partition. The reason is that the /etc/rc.d/mdconfig script is executed very early during boot and the root partition is still read-only. Images located outside the root partition will be mounted later with the script /etc/rc.d/mdconfig2./boot/loader.conf:

md_load="YES"

/etc/rc.conf:

# mdconfig_md0="-t vnode -f /usr/vdisk.img" # /usr is not on the root partition

/etc/fstab: (The 0 0 at the end is important, it tell fsck to ignore this device, as is does not exist yet)

/dev/md0 /usr/vdisk ufs rw 0 0

It is also possible to increase the size of the image afterward, say for example 300 MB larger.

Delete the port forward with -D instead of -A. The program netstat-nathttp://tweegy.nl/projects/netstat-nat is very useful to track connections (it uses /proc/net/ip_conntrack or /proc/net/nf_conntrack).

Forward queries

Dig is you friend to test the DNS settings. For example the public DNS server 213.133.105.2 ns.second-ns.de can be used for testing. See from which server the client receives the answer (simplified answer).

On Windows use windump from www.winpcap.org. Use windump -D to list the interfaces.

Scan with nmap

Nmaphttp://insecure.org/nmap/ is a port scanner with OS detection, it is usually installed on most distributions and is also available for Windows. If you don’t scan your servers, hackers do it for you…

Other non standard but useful tools are hping (www.hping.org) an IP packet assembler/analyzer and fping (fping.sourceforge.net). fping can check multiple hosts in a round-robin fashion.

Traffic control (QoS)

Traffic control manages the queuing, policing, scheduling, and other traffic parameters for a network. The following examples are simple practical uses of the Linux and FreeBSD capabilities to better use the available bandwidth.

Limit upload

DSL or cable modems have a long queue to improve the upload throughput. However filling the queue with a fast device (e.g. ethernet) will dramatically decrease the interactivity. It is therefore useful to limit the device upload rate to match the physical capacity of the modem, this should greatly improve the interactivity. Set to about 90% of the modem maximal (cable) speed.

FreeBSD

FreeBSD uses the dummynet traffic shaper which is configured with ipfw. Pipes are used to set limits the bandwidth in units of [K|M]{bit/s|Byte/s}, 0 means unlimited bandwidth. Using the same pipe number will reconfigure it. For example limit the upload bandwidth to 500 Kbit.

Quality of service

Linux

Priority queuing with tc to optimize VoIP. See the full example on voip-info.org or www.howtoforge.com. Suppose VoIP uses udp on ports 10000:11024 and device eth0 (could also be ppp0 or so). The following commands define the QoS to three queues and force the VoIP traffic to queue 1 with QoS 0x1e (all bits set). The default traffic flows into queue 3 and QoS Minimize-Delay flows into queue 2.

Calculate port range and mask

The tc filter defines the port range with port and mask which you have to calculate. Find the 2^N ending of the port range, deduce the range and convert to HEX. This is your mask. Example for 10000 -> 11024, the range is 1024.

Linux

# cat /etc/yp.conf
ypserver servername
domain domain.net broadcast

Netcat

Netcathttp://netcat.sourceforge.net (nc) is better known as the “network Swiss Army Knife”, it can manipulate, create or read/write TCP/IP connections. Here some useful examples, there are many more on the net, for example g-loaded.eu[…]http://www.g-loaded.eu/2006/11/06/netcat-a-couple-of-useful-examples and herehttp://www.terminally-incoherent.com/blog/2007/08/07/few-useful-netcat-tricks.You might need to use the command netcat instead of nc. Also see the similar command socat.

File transfer

Copy a large folder over a raw tcp connection. The transfer is very quick (no protocol overhead) and you don’t need to mess up with NFS or SMB or FTP or so, simply make the file available on the server, and get it from the client. Here 192.168.1.1 is the server IP address.

Chat

See other tricks 25 ssh cmdhttp://blog.urfix.com/25-ssh-commands-tricks/

Public key authentication

Connect to a host without password using public key authentication. The idea is to append your public key to the authorized_keys2 file on the remote host. For this example let’s connect host-client to host-server, the key is generated on the client. With cygwin you might have to create your home directoy and the .ssh directory with # mkdir -p /home/USER/.ssh

Use ssh-keygen to generate a key pair. ~/.ssh/id_dsa is the private key, ~/.ssh/id_dsa.pub is the public key.

Copy only the public key to the server and append it to the file ~/.ssh/authorized_keys2 on your home on the server.

Using the Windows client from ssh.com

The non commercial version of the ssh.com client can be downloaded the main ftp site: ftp.ssh.com/pub/ssh/. Keys generated by the ssh.com client need to be converted for the OpenSSH server. This can be done with the ssh-keygen command.

Check fingerprint

At the first login, ssh will ask if the unknown host with the fingerprint has to be stored in the known hosts. To avoid a man-in-the-middle attack the administrator of the server can send you the server fingerprint which is then compared on the first login. Use ssh-keygen -l to get the fingerprint (on the server):

Secure file transfer

In Konqueror or Midnight Commander it is possible to access a remote file system with the address fish://user@gate. However the implementation is very slow.Furthermore it is possible to mount a remote folder with sshfs a file system client based on SCP. See fuse sshfshttp://fuse.sourceforge.net/sshfs.html.

ssh_exchange_identification: Connection closed by remote host

With this error try the following on the server:

echo 'SSHD: ALL' >> /etc/hosts.allow
/etc/init.d/sshd restart

Tunneling

SSH tunneling allows to forward or reverse forward a port over the SSH connection, thus securing the traffic and accessing ports which would otherwise be blocked. This only works with TCP. The general nomenclature for forward and reverse is (see also ssh and NAT example):

This will connect to gate and forward the local port to the host desthost:destport. Note desthost is the destination host as seen by the gate, so if the connection is to the gate, then desthost is localhost. More than one port forward is possible.

Direct forward on the gate

Let say we want to access the CVS (port 2401) and http (port 80) which are running on the gate. This is the simplest example, desthost is thus localhost, and we use the port 8080 locally instead of 80 so we don’t need to be root. Once the ssh session is open, both services are accessible on the local ports.

# ssh -L 2401:localhost:2401 -L 8080:localhost:80 user@gate

Netbios and remote desktop forward to a second server

Let say a Windows smb server is behind the gate and is not running ssh. We need access to the smb share and also remote desktop to the server.

# ssh -L 139:smbserver:139 -L 3388:smbserver:3389 user@gate

The smb share can now be accessed with \\127.0.0.1\, but only if the local share is disabled, because the local share is listening on port 139.It is possible to keep the local share enabled, for this we need to create a new virtual device with a new IP address for the tunnel, the smb share will be connected over this address. Furthermore the local RDP is already listening on 3389, so we choose 3388. For this example let’s use a virtual IP of 10.1.1.1.

With putty use Source port=10.1.1.1:139. It is possible to create multiple loop devices and tunnel. On Windows 2000, only putty worked for me. On Windows Vista also forward the port 445 in addition to the port 139. Also on Vista the patch KB942624 prevents the port 445 to be forwarded, so I had to uninstall this path in Vista.

With the ssh.com client, disable “Allow local connections only”. Since ssh.com will bind to all addresses, only a single share can be connected.

I HAD to reboot for this to work. Now connect to the smb share with \\10.1.1.1 and remote desktop to 10.1.1.1:3388.

Debug

If it is not working:

Are the ports forwarded: netstat -an? Look at 0.0.0.0:139 or 10.1.1.1:139

Does telnet 10.1.1.1 139 connect?

You need the checkbox “Local ports accept connections from other hosts”.

Is “File and Printer Sharing for Microsoft Networks” disabled on the loopback interface?

Connect two clients behind NAT

Suppose two clients are behind a NAT gateway and client cliadmin has to connect to client cliuser (the destination), both can login to the gate with ssh and are running Linux with sshd. You don’t need root access anywhere as long as the ports on gate are above 1024. We use 2022 on gate. Also since the gate is used locally, the option GatewayPorts is not necessary.On client cliuser (from destination to gate):

Connect to VNC behind NAT

Suppose a Windows client with VNC listening on port 5900 has to be accessed from behind NAT.On client cliwin to gate:

# ssh -R 15900:localhost:5900 user@gate

On client cliadmin (from host to gate):

# ssh -L 5900:localhost:15900 admin@gate

Now the admin can connect directly to the client VNC with:

# vncconnect -display :0 localhost

Dig a multi-hop ssh tunnel

Suppose you can not reach a server directly with ssh, but only via multiple intermediate hosts (for example because of routing issues). Sometimes it is still necessary to get a direct client – server connection, for example to copy files with scp, or forward other ports like smb or vnc. One way to do this is to chain tunnels together to forward a port to the server along the hops. This “carrier” port only reaches its final destination on the last connection to the server.Suppose we want to forward the ssh port from a client to a server over two hops. Once the tunnel is build, it is possible to connect to the server directly from the client (and also add an other port forward).

Create tunnel in one shell

client -> host1 -> host2 -> server and dig tunnel 5678

client># ssh -L5678:localhost:5678 host1 # 5678 is an arbitrary port for the tunnelhost_1># ssh -L5678:localhost:5678 host2 # chain 5678 from host1 to host2host_2># ssh -L5678:localhost:22 server # end the tunnel on port 22 on the server

As of version 4.3, OpenSSH can use the tun/tap device to encrypt a tunnel. This is very similar to other TLS based VPN solutions like OpenVPN. One advantage with SSH is that there is no need to install and configure additional software. Additionally the tunnel uses the SSH authentication like pre shared keys. The drawback is that the encapsulation is done over TCP which might result in poor performance on a slow link. Also the tunnel is relying on a single (fragile) TCP connection. This technique is very useful for a quick IP based VPN setup. There is no limitation as with the single TCP port forward, all layer 3/4 protocols like ICMP, TCP/UDP, etc. are forwarded over the VPN. In any case, the following options are needed in the sshd_conf file:

PermitRootLogin yes
PermitTunnel yes

Single P2P connection

Here we are connecting two hosts, hclient and hserver with a peer to peer tunnel. The connection is started from hclient to hserver and is done as root. The tunnel end points are 10.0.1.1 (server) and 10.0.1.2 (client) and we create a device tun5 (this could also be an other number). The procedure is very simple:

Connect with SSH using the tunnel option -w

Configure the IP addresses of the tunnel. Once on the server and once on the client.

Connect to the server

Connection started on the client and commands are executed on the server.

Configure the client

The two hosts are now connected and can transparently communicate with any layer 3/4 protocol using the tunnel IP addresses.

Connect two networks

In addition to the p2p setup above, it is more useful to connect two private networks with an SSH VPN using two gates. Suppose for the example, netA is 192.168.51.0/24 and netB 192.168.16.0/24. The procedure is similar as above, we only need to add the routing. NAT must be activated on the private interface only if the gates are not the same as the default gateway of their network.192.168.51.0/24 (netA)|gateA <-> gateB|192.168.16.0/24 (netB)

Connect with SSH using the tunnel option -w.

Configure the IP addresses of the tunnel. Once on the server and once on the client.

The two private networks are now transparently connected via the SSH VPN. The IP forward and NAT settings are only necessary if the gates are not the default gateways. In this case the clients would not know where to forward the response, and nat must be activated.

Rsync can almost completely replace cp and scp, furthermore interrupted transfers are efficiently restarted. A trailing slash (and the absence thereof) has different meanings, the man page is good… Here some examples:Copy the directories with full content:

Exclude any directory tmp within /home/user/ and keep the relative folders hierarchy, that is the remote directory will have the structure /backup/home/user/. This is typically used for backups.

# rsync -azR --exclude=tmp/ /home/user/ user@server:/backup/

Use port 20022 for the ssh connection:

# rsync -az -e 'ssh -p 20022' /home/colin/ user@server:/backup/colin/

Using the rsync daemon (used with “::”) is much faster, but not encrypted over ssh. The location of /backup is defined by the configuration in /etc/rsyncd.conf. The variable RSYNC_PASSWORD can be set to avoid the need to enter the password manually.

Rsync on Windows

Rsync is available for Windows through cygwin or as stand-alone packaged in cwrsynchttp://sourceforge.net/projects/sereds. This is very convenient for automated backups. Install one of them (not both) and add the path to the Windows system variables: # Control Panel -> System -> tab Advanced, button Environment Variables. Edit the “Path” system variable and add the full path to the installed rsync, e.g. C:\Program Files\cwRsync\bin or C:\cygwin\bin. This way the commands rsync and ssh are available in a Windows command shell.

Public key authentication

Rsync is automatically tunneled over SSH and thus uses the SSH authentication on the server. Automatic backups have to avoid a user interaction, for this the SSH public key authentication can be used and the rsync command will run without a password.All the following commands are executed within a Windows console. In a console (Start -> Run -> cmd) create and upload the key as described in SSH, change “user” and “server” as appropriate. If the file authorized_keys2 does not exist yet, simply copy id_dsa.pub to authorized_keys2 and upload it.

# ssh-keygen -t dsa -N '' # Creates a public and a private key
# rsync user@server:.ssh/authorized_keys2 . # Copy the file locally from the server
# cat id_dsa.pub >> authorized_keys2 # Or use an editor to add the key
# rsync authorized_keys2 user@server:.ssh/ # Copy the file back to the server
# del authorized_keys2 # Remove the local copy

Automatic backup

Use a batch file to automate the backup and add the file in the scheduled tasks (Programs -> Accessories -> System Tools -> Scheduled Tasks). For example create the file backup.bat and replace user@server.

@ECHO OFF
REM rsync the directory My Documents
SETLOCAL
SET CWRSYNCHOME=C:\PROGRAM FILES\CWRSYNC
SET CYGWIN=nontsec
SET CWOLDPATH=%PATH%
REM uncomment the next line when using cygwin
SET PATH=%CWRSYNCHOME%\BIN;%PATH%
echo Press Control-C to abort
rsync -av "/cygdrive/c/Documents and Settings/%USERNAME%/My Documents/" \
'user@server:My\ Documents/'
pause

Sudo is a standard way to give users some administrative rights without giving out the root password. Sudo is very useful in a multi user environment with a mix of server and workstations. Simply call the command with sudo:

# sudo /etc/init.d/dhcpd restart # Run the rc script as root
# sudo -u sysadmin whoami # Run cmd as an other user

Configuration

Sudo is configured in /etc/sudoers and must only be edited with visudo. The basic syntax is (the lists are comma separated):

user hosts = (runas) commands # In /etc/sudoers

users one or more users or %group (like %wheel) to gain the rights

hosts list of hosts (or ALL)

runas list of users (or ALL) that the command rule can be run as. It is enclosed in ( )!

commands list of commands (or ALL) that will be run as root or as (runas)

Additionally those keywords can be defined as alias, they are called User_Alias, Host_Alias, Runas_Alias and Cmnd_Alias. This is useful for larger setups. Here a sudoers example:

# The actual rules
root,ADMINS ALL = (ALL) NOPASSWD: ALL # ADMINS can do anything w/o a password.
DEVEL DESKTOP = (ALL) NOPASSWD: ALL # Developers have full right on desktops
DEVEL DMZ = (ALL) NOPASSWD: DEBUG # Developers can debug the DMZ servers.# User sysadmin can mess around in the DMZ servers with some commands.
sysadmin DMZ = (ALL) NOPASSWD: SYSTEM,PW,DEBUG
sysadmin ALL,!DMZ = (ALL) NOPASSWD: ALL # Can do anything outside the DMZ.
%dba ALL = (DBA) ALL # Group dba can run as database user.# anyone can mount/unmount a cd-rom on the desktop machines
ALL DESKTOP = NOPASSWD: /sbin/mount /cdrom,/sbin/umount /cdrom

tar zip and encrypt a whole directory

Use -k mysecretpassword after aes-128-cbc to avoid the interactive password request. However note that this is highly insecure.

Use aes-256-cbc instead of aes-128-cbc to get even stronger encryption. This uses also more CPU.

GPG

GnuPG is well known to encrypt and sign emails or any data. Furthermore gpg and also provides an advanced key management system. This section only covers files encryption, not email usage, signing or the Web-Of-Trust.The simplest encryption is with a symmetric cipher. In this case the file is encrypted with a password and anyone who knows the password can decrypt it, thus the keys are not needed. Gpg adds an extention “.gpg” to the encrypted file names.

Using keys

For more details see GPG Quick Starthttp://www.madboa.com/geek/gpg-quickstart and GPG/PGP Basicshttp://aplawrence.com/Basics/gpg.html and the gnupg documentationhttp://gnupg.org/documentation among others.The private and public keys are the heart of asymmetric cryptography. What is important to remember:

Your public key is used by others to encrypt files that only you as the receiver can decrypt (not even the one who encrypted the file can decrypt it). The public key is thus meant to be distributed.

Your private key is encrypted with your passphrase and is used to decrypt files which were encrypted with your public key. The private key must be kept secure. Also if the key or passphrase is lost, so are all the files encrypted with your public key.

The key files are called keyrings as they can contain more than one key.

First generate a key pair. The defaults are fine, however you will have to enter at least your full name and email and optionally a comment. The comment is useful to create more than one key with the same name and email. Also you should use a “passphrase”, not a simple password.

# gpg --gen-key # This can take a long time

The keys are stored in ~/.gnupg/ on Unix, on Windows they are typically stored inC:/Documents and Settings/%USERNAME%/Application Data/gnupg/.

~/.gnupg/pubring.gpg # Contains your public keys and all others imported
~/.gnupg/secring.gpg # Can contain more than one private key

Short reminder on most used options:

-e encrypt data

-d decrypt data

-r NAME encrypt for recipient NAME (or ‘Full Name’ or ’email@domain’)

-a create ascii armored output of a key

-o use as output file

The examples use ‘Your Name’ and ‘Alice’ as the keys are referred to by the email or full name or partial name. For example I can use ‘Colin’ or ‘c@cb.vu’ for my key [Colin Barschel (cb.vu) <c@cb.vu>].

Encrypt for personal use only

Encrypt – Decrypt with keys

First you need to export your public key for someone else to use it. And you need to import the public say from Alice to encrypt a file for her. You can either handle the keys in simple ascii files or use a public key server.For example Alice export her public key and you import it, you can then encrypt a file for her. That is only Alice will be able to decrypt it.

There are (many) other alternative methods to encrypt disks, I only show here the methods I know and use. Keep in mind that the security is only good as long the OS has not been tempered with. An intruder could easily record the password from the keyboard events. Furthermore the data is freely accessible when the partition is attached and will not prevent an intruder to have access to it in this state.

Linux

Those instructions use the Linux dm-crypt (device-mapper) facility available on the 2.6 kernel. In this example, lets encrypt the partition /dev/sdc1, it could be however any other partition or disk, or USB or a file based partition created with losetup. In this case we would use /dev/loop0. See file image partition. The device mapper uses labels to identify a partition. We use sdc1 in this example, but it could be any string.

dm-crypt with LUKS

LUKS with dm-crypt has better encryption and makes it possible to have multiple passphrase for the same partition or to change the password easily. To test if LUKS is available, simply type # cryptsetup --help, if nothing about LUKS shows up, use the instructions below Without LUKS. First create a partition if necessary: fdisk /dev/sdc.

Do exactly the same (without the mkfs part!) to re-attach the partition. If the password is not correct, the mount command will fail. In this case simply remove the map sdc1 (cryptsetup remove sdc1) and create it again.

FreeBSD

The two popular FreeBSD disk encryption modules are gbde and geli. I now use geli because it is faster and also uses the crypto device for hardware acceleration. See The FreeBSD handbook Chapter 18.6http://www.freebsd.org/handbook/disks-encrypting.html for all the details. The geli module must be loaded or compiled into the kernel:

Use password and key

I use those settings for a typical disk encryption, it uses a passphrase AND a key to encrypt the master key. That is you need both the password and the generated key /root/ad1.key to attach the partition. The master key is stored inside the partition and is not visible. See below for typical USB or file based image.

Use password only

It is more convenient to encrypt a USB stick or file based image with a passphrase only and no key. In this case it is not necessary to carry the additional key file around. The procedure is very much the same as above, simply without the key file. Let’s encrypt a file based image /cryptedfile of 1 GB.

So called SSL/TLS certificates are cryptographic public key certificates and are composed of a public and a private key. The certificates are used to authenticate the endpoints and encrypt the data. They are used for example on a web server (https) or mail server (imaps).

Procedure

We need a certificate authority to sign our certificate. This step isusually provided by a vendor like Thawte, Verisign, etc., however we can also create our own.

Create a certificate signing request. This request is like an unsigned certificate (the public part) and already contains all necessary information. The certificate request is normally sent to the authority vendor for signing. This step also creates the private key on the local machine.

Sign the certificate with the certificate authority.

If necessary join the certificate and the key in a single file to be used by the application (web server, mail server etc.).

Configure OpenSSL

We use /usr/local/certs as directory for this example check or edit /etc/ssl/openssl.cnf accordingly to your settings so you know where the files will be created. Here are the relevant part of openssl.cnf:

If you intend to get a signed certificate from a vendor, you only need a certificate signing request (CSR). This CSR will then be signed by the vendor for a limited time (e.g. 1 year).

Create a certificate authority

If you do not have a certificate authority from a vendor, you’ll have to create your own. This step is not necessary if one intend to use a vendor to sign the request. To make a certificate authority (CA):

Create a certificate signing request

To make a new certificate (for mail server or web server for example), first create a request certificate with its private key. If your application do not support encrypted private key (for example UW-IMAP does not), then disable encryption with -nodes.

Now servernamekey.pem is the private key and servernamecert.pem is the server certificate.

Create united certificate

The IMAP server wants to have both private key and server certificate in the same file. And in general, this is also easier to handle, but the file has to be kept securely!. Apache also can deal with it well. Create a file servername.pem containing both the certificate and key.

Open the private key (servernamekey.pem) with a text editor and copy the private key into the “servername.pem” file.

Add a readers file if you want to differentiate read and write permissions Note: Do not (ever) edit files directly into the main cvs, but rather checkout the file, modify it and check it in. We did this with the file writers to define the write access.There are three popular ways to access the CVS at this point. The first two don’t need any further configuration. See the examples on CVSROOT below for how to use them:

Direct local access to the file system. The user(s) need sufficient file permission to access the CS directly and there is no further authentication in addition to the OS login. However this is only useful if the repository is local.

Remote access with ssh with the ext protocol. Any use with an ssh shell account and read/write permissions on the CVS server can access the CVS directly with ext over ssh without any additional tunnel. There is no server process running on the CVS for this to work. The ssh login does the authentication.

Remote access with pserver (default port: 2401/tcp). This is the preferred use for larger user base as the users are authenticated by the CVS pserver with a dedicated password database, there is therefore no need for local users accounts. This setup is explained below.

Network setup with inetd

The CVS can be run locally only if a network access is not needed. For a remote access, the daemon inetd can start the pserver with the following line in /etc/inetd.conf (/etc/xinetd.d/cvs on SuSE):

It is a good idea to block the cvs port from the Internet with the firewall and use an ssh tunnel to access the repository remotely.

Separate authentication

It is possible to have cvs users which are not part of the OS (no local users). This is actually probably wanted too from the security point of view. Simply add a file named passwd (in the CVSROOT directory) containing the users login and password in the crypt format. This is can be done with the apache htpasswd tool.Note: This passwd file is the only file which has to be edited directly in the CVSROOT directory. Also it won’t be checked out. More info with htpasswd –help

Test it

CVSROOT variable

This is an environment variable used to specify the location of the repository we’re doing operations on. For local use, it can be just set to the directory of the repository. For use over the network, the transport protocol must be specified. Set the CVSROOT variable with setenv CVSROOT string on a csh, tcsh shell, or with export CVSROOT=string on a sh, bash shell.

CVS commands and usage

Import

The import command is used to add a whole directory, it must be run from within the directory to be imported. Say the directory /devel/ contains all files and subdirectories to be imported. The directory name on the CVS (the module) will be called “myapp”.

Subversion (SVN)http://subversion.tigris.org/ is a version control system designed to be the successor of CVS (Concurrent Versions System). The concept is similar to CVS, but many shortcomings where improved. See also the SVN bookhttp://svnbook.red-bean.com/en/1.4/.

Server setup

The initiation of the repository is fairly simple (here for example /home/svn/ must exist):

# svnadmin create --fs-type fsfs /home/svn/project1

Now the access to the repository is made possible with:

file:// Direct file system access with the svn client with. This requires local permissions on the file system.

svn:// or svn+ssh:// Remote access with the svnserve server (also over SSH). This requires local permissions on the file system (default port: 2690/tcp).

http:// Remote access with webdav using apache. No local users are necessary for this method.

Using the local file system, it is now possible to import and then check out an existing project. Unlike with CVS it is not necessary to cd into the project directory, simply give the full path:

Remote access with ssh

No special setup is required to access the repository via ssh, simply replace file:// with svn+ssh/hostname. For example:

# svn checkout svn+ssh://hostname/home/svn/project1

As with the local file access, every user needs an ssh access to the server (with a local account) and also read/write access. This method might be suitable for a small group. All users could belong to a subversion group which owns the repository, for example:

less

The less command displays a text document on the console. It is present on most installation.

# less unixtoolbox.xhtml

Some important commands are (^N stands for [control]-[N]):

h H good help on display

f ^F ^V SPACE Forward one window (or N lines).

b ^B ESC-v Backward one window (or N lines).

F Forward forever; like “tail -f”.

/pattern Search forward for (N-th) matching line.

?pattern Search backward for (N-th) matching line.

n Repeat previous search (for N-th occurrence).

N Repeat previous search in reverse direction.

q quit

vi

Vi is present on ANY Linux/Unix installation (not gentoo?) and it is therefore useful to know some basic commands. There are two modes: command mode and insertion mode. The commands mode is accessed with [ESC], the insertion mode with i. Use : help if you are lost.The editors nano and pico are usually available too and are easier (IMHO) to use.

Quit

:w newfilename save the file to newfilename

:wq or 😡 save and quit

:q! quit without saving

Search and move

/string Search forward for string

?string Search back for string

n Search for next instance of string

N Search for previous instance of string

{ Move a paragraph back

} Move a paragraph forward

1G Move to the first line of the file

nG Move to the n th line of the file

G Move to the last line of the file

:%s/OLD/NEW/g Search and replace every occurrence

Delete copy paste text

dd (dw) Cut current line (word)

D Cut to the end of the line

x Delete (cut) character

yy (yw) Copy line (word) after cursor

P Paste after cursor

u Undo last modification

U Undo all changes to current line

mail

The mail command is a basic application to read and send email, it is usually installed. To send an email simply type “mail user@domain”. The first line is the subject, then the mail content. Terminate and send the email with a single dot (.) in a new line. Example:

# mail c@cb.vu
Subject: Your text is full of typos
"For a moment, nothing happened. Then, after a second or so,
nothing continued to happen."
.
EOT
#

This is also working with a pipe:

# echo "This is the mail body" | mail c@cb.vu

This is also a simple way to test the mail server.

tar

The command tar (tape archive) creates and extracts archives of file and directories. The archive .tar is uncompressed, a compressed archive has the extension .tgz or .tar.gz (zip) or .tbz (bzip2). Do not use absolute path when creating an archive, you probably want to unpack it somewhere else. Some typical commands are:

notrunc do not truncate the output file, all zeros will be written as zeros.

noerror continue after read errors (e.g. bad blocks)

sync pad every input block with Nulls to ibs-size

The default byte size is 512 (one block). The MBR, where the partition table is located, is on the first block, the first 63 blocks of a disk are empty. Larger byte sizes are faster to copy but require also more memory.

Recover

The command dd will read every single block of the partition. In case of problems it is better to use the option conv=sync,noerror so dd will skip the bad block and write zeros at the destination. Accordingly it is important to set the block size equal or smaller than the disk block size. A 1k size seems safe, set it with bs=1k. If a disk has bad sectors and the data should be recovered from a partition, create an image file with dd, mount the image and copy the content to a new disk. With the option noerror, dd will skip the bad sectors and write zeros instead, thus only the data contained in the bad sectors will be lost.

Be careful with xarg or exec as it might or might not honor quotings and can return wrong results when files or directories contain spaces. In doubt use “-print0 | xargs -0” instead of “| xargs”. The option -print0 must be the last in the find command. See this nice mini tutorial for findhttp://www.hccfl.edu/pollock/Unix/FindCmd.htm.

Library path

Due to complex dependencies and runtime linking, programs are difficult to copy to an other system or distribution. However for small programs with little dependencies, the missing libraries can be copied over. The runtime libraries (and the missing one) are checked with ldd and managed with ldconfig.

Without the -f option, iconv will use the local char-set, which is usually fineif the document displays well.Convert filenames from one encoding to another (not file content). Works also if only some files are already utf8

# convmv -r -f utf8 --nfd -t utf8 --nfc /dir/* --notest

Unix – DOS newlines

Convert DOS (CR/LF) to Unix (LF) newlines and back within a Unix shell. See also dos2unix and unix2dos if you have them.

PostgreSQL

Change root or a username password

Create user and database

The commands createuser, dropuser, createdb and dropdb are convenient shortcuts equivalent to the SQL commands. The new user is bob with database bobdb ; use as root with pgsql the database super user:

Grant remote access

Remote access is typically permitted for a database, and not all databases. The file /etc/my.cnf contains the IP address to bind to. (On FreeBSD my.cnf not created per fedault, copy one .cnf file from /usr/local/share/mysql to /usr/local/etc/my.cnf) Typically comment the line bind-address = out.

# mysql -u root mysql
mysql> GRANT ALL ON bobdb.* TO bob@'xxx.xxx.xxx.xxx' IDENTIFIED BY 'PASSWORD';
mysql> REVOKE GRANT OPTION ON foo.* FROM bar@'xxx.xxx.xxx.xxx';
mysql> FLUSH PRIVILEGES; # Use 'hostname' or also '%' for full access

Here is “secret” the mysql root password, there is no space after -p. When the -p option is used alone (w/o password), the password is asked at the command prompt.

SQLite

SQLitehttp://www.sqlite.org is a small powerful self-contained, serverless, zero-configuration SQL database.

Dump and restore

It can be useful to dump and restore an SQLite database. For example you can edit the dump file to change a column attribute or type and then restore the database. This is easier than messing with SQL commands. Use the command sqlite3 for a 3.x database.

Convert 2.x to 3.x database

A disk quota allows to limit the amount of disk space and/or the number of files a user or (or member of group) can use. The quotas are allocated on a per-file system basis and are enforced by the kernel.

Linux setup

The quota tools package usually needs to be installed, it contains the command line tools.Activate the user quota in the fstab and remount the partition. If the partition is busy, either all locked files must be closed, or the system must be rebooted. Add usrquota to the fstab mount options, for example:

Assign quota limits

The quotas are not limited per default (set to 0). The limits are set with edquota for single users. A quota can be also duplicated to many users. The file structure is different between the quota implementations, but the principle is the same: the values of blocks and inodes can be limited. Only change the values of soft and hard. If not specified, the blocks are 1k. The grace period is set with edquota -t. For example:

Modify your configuration in ~/.bashrc (it can also be ~/.bash_profile). The following entries are useful, reload with “. .bashrc”. With cygwin use ~/.bash_profile; with rxvt past with shift + left-click.

# in .bashrc
bind '"\e[A"':history-search-backward # Use up and down arrow to search
bind '"\e[B"':history-search-forward # the history. Invaluable!
set -o emacs # Set emacs mode in bash (see below)
set bell-style visible # Do not beep, inverse colors# Set a nice prompt like [user@host]/path/todir>
PS1="\[\033[1;30m\][\[\033[1;34m\]\u\[\033[1;30m\]"
PS1="$PS1@\[\033[0;33m\]\h\[\033[1;30m\]]\[\033[0;37m\]"
PS1="$PS1\w\[\033[1;30m\]>\[\033[0m\]"

Regular Expressions

Some basic regular expression useful for sed too. See Basic Regex Syntaxhttp://www.regular-expressions.info/reference.html for a good primer.

[\^$.|?*+() # special characters any other will match themselves
\ # escapes special characters and treat as literal
* # repeat the previous item zero or more times
. # single character except line break characters
.* # match zero or more characters
^ # match at the start of a line/string
$ # match at the end of a line/string
.$ # match a single character at the end of line/string
^ $ # match line with a single space
^[A-Z] # match any line beginning with any char from A to Z

Some useful commands

The following commands are useful to include in a script or as one liners.

I use this little trick to change the file extension for many files at once. For example from .cxx to .cpp. Test it first without the | sh at the end. You can also do this with the command rename if installed. Or with bash builtins.

C example

C++ basics

*pointer // Object pointed to by pointer
&obj // Address of object obj
obj.x // Member x of class obj (object obj)
pobj->x // Member x of class pointed to by pobj// (*pobj).x and pobj->x are the same

C++ example

As a slightly more realistic program in C++: a class in its own header (IPv4.h) and implementation (IPv4.cpp) and a program which uses the class functionality. The class converts an IP address in integer format to the known quad format.